BACKGROUND AND OBJECTIVES: Since 1903,
blood pressure has been noninvasively monitored (NIBP), either with manual sphygmomanometer
or automated noninvasive devices. One NIBP measurement problem is the considerable
variance in blood pressure data, both within and between available techniques.
The oscillometric method for NIBP monitoring evaluates blood pressure during
cuff deflation. Difficulties in blood pressure measurement by oscillometry may
arise from: inadequate cuff size, inadequate cuff application, undetected fails
in cuff, hoses, or connectors, arm movement, shock and vascular compression
proximal to the cuff. This study aimed at evaluating the reliability of three
noninvasive blood pressure monitoring devices during five measurements.METHODS: Blood pressure of 60 healthy female volunteers aged 20 to 40
years was evaluated from 7 am to 11 am, in the sitting position during a normal
workday. Five measures were taken with each device at 2-minute intervals. Three
automatic blood pressure monitors were studied. No patient was obese, hypertensive
or suffering from cardiac disease and cardiac arrhythmia. Indirect measurements
were made according to manufacturers' instructions.RESULTS: There were no differences in demographics among the three studied
groups. Mean intrapersonal variation from one measurement to the other was up
to 6.7 mmHg for systolic blood pressure (SBP), 4.9 mmHg for mean blood pressure
(MBP) and 3.3 mmHg for diastolic blood pressure (DBP) with 95% confidence interval.
The highest difference between measures in the same volunteer was 49 mmHg for
SBP, 46 mmHg for MBP and 28 mmHg for DBP.CONCLUSIONS: This study has shown significant variations in SBP, MBP
and DBP and that SBP is the most reliable parameter to check blood pressure
changes in volunteers.

The difference in blood pressure among vessels
was firstly referred to by the Renascent painter Giovanni Di Paolo in representing
St. John Baptist's decapitation with some vessels pouring blood (arteries) and
others dripping blood (veins) 1. Riva-Rocci, in 1896, has described
in two successive publications a new mercury column sphygmomanometer to evaluate
blood pressure variations 2,3.

The standardization of technical procedures is
important for epidemiologic studies involving different observers because systematic
errors and especially observer-introduced errors, especially in blood pressure
which has intrinsic variations, may compromise results. Routine blood pressure
monitoring is still not standardized, almost always not following basic recommendations
to avoid reading errors 4,5. Several factors related to equipment,
observer, environment, patient and the technique itself, may interfere with
blood pressure measurement accuracy. Criteria to be followed during NIBP measurement
are clearly stated in our country 6. The strict adherence to the
correct blood pressure measurement procedure should be adopted not only in the
clinical practice but especially in clinical investigations and experiments
including blood pressure measurement in their methods. In evaluating blood pressure
measurement reported in the literature, it has been observed that, from a list
of 19 items to be followed, only 20% have followed 12 of them 5.
In a different study analyzing 223 studies published by 18 Brazilian journals
related to clinical practice in the period 1989-1994, it was observed that half
the studies have not specified the type of sphygmomanometer used 7.
In studies where the type of device was mentioned, 29% were mercury column,
16% aneroids and 4% electronic 7. In addition, calibration checks
were not mentioned in 82%, only 18% have expressed concern in checking system's
accuracy and 25% of the studies have mentioned measurements position 7.
The Brazilian Journal of Anesthesiology has published 274 studies from 1999
to 2002, of which 118 have studied blood pressure and none has mentioned the
device or its accuracy. From those 274 studies, 15 have studied hemodynamic
changes during spinal anesthesia. In a study on the efficacy of ephedrine to
prevent arterial hypotension during Cesarean sections, the devices were also
not mentioned 8,9. The identification of patients at higher risk
for arterial hypotension during spinal anesthesia has already been studied where
predicting factors were considered age above 45 years, female gender and sensory
block level above T7 10.

Our study has evaluated three noninvasive blood
pressure measurement devices commonly used in operating rooms and anesthesia
machines, in an investigation project with volunteers aged 20 to 40 years without
history of arterial hypotension or use of medication, with the objective of
evaluating the reliability of intermittent blood pressure measurements.

METHODS

Participated in this study 60 female volunteers,
aged 20 to 40 years (assistant nurses, nurses, instrumentation technicians and
physicians) working at surgical centers of Clinica São Bernardo and Hospital
Barra D'Or in Rio de Janeiro, and Hospital de Base from São José
do Rio Preto in the period 7 a.m. to 11 a.m. of a normal workday. All volunteers
had normal blood pressures and were not under any anti-hypertensive medication,
were not obese and had no history of arrhythmias. Volunteers with arterial hypertension
during gestation were excluded from the study. Blood pressure was measured five
times at 2-minute intervals, in surgical center rooms and according to Brazilian
Consensus recommendations for arterial hypertension treatment 6.
All measures were obtained in the sitting position and with right arm maintained
at the level of the heart. It has been asked whether the 2-minute interval adopted
would bring any discomfort.

Three devices were used in the study: Anamed-Vital
Line AM 78100B, Hewlett-Packard (HP) model Viridia 24 C and Dixtal DX 2010.
Indirect blood pressure was obtained according to manufacturers' instructions.

Repeated measures analysis method was used to
evaluate effects of time, device (profession) and interaction between time and
device (time and profession). Analysis of variance was used to compare age,
weight, 1st to 5th measures variation and height according
to device (profession). Paired t test was used to compare general variations
among measures and confidence intervals for mean variations were determined
by normal distribution, since all were approved by the normality test.

RESULTS

There have been no differences in mean age, weight
and height, or according to device of profession.

Device and Time Comparison

Systolic blood pressure (SBP): there is
evidence of time effect (p = 0.017) with intrapersonal mean decreasing with
time; there is no evidence of device effect or of interaction between time and
device (p = 0.36 and p = 0.84 respectively) (Figure
1).

Diastolic blood pressure (DBP): there
is evidence of device effect (HP with mean values lower than other two brands)
(p = 0.000); there is no evidence of time effect or of interaction between time
and device (p = 0.12 and p = 0.68 respectively) (Figure
2).

Mean blood pressure (MBP): there is evidence
of device effect (p = 0.000) with HP showing lower mean values as compared to
other brands. There is evidence of time effect (p = 0.041) with intrapersonal
mean decreasing along time. There is no evidence of interaction between time
and device (p = 0.82) (Figure
3).

Profession and Time Comparison

Systolic blood pressure (SBP): there is
evidence of profession effect (p = 0.000), with lower mean values for instrumentation
technician as compared to other professions. There is evidence of time effect
(p = 0.014) with intrapersonal mean decreasing along time. There is no evidence
of interaction between time and profession (p = 0.47) (Figure
4).

Diastolic blood pressure (DBP): there
is evidence of profession effect (p = 0.000), with lower mean values for instrumentation
technique as compared to assistant nurse and physician. There is no evidence
of time effect (p = 0.14) or of interaction between time and profession (p =
0.98) (Figure 5).

Mean blood pressure (MBP): there is evidence
of profession effect (p = 0.000) with lower mean values for instrumentation
technician as compared to assistant nurse and physician, and there is evidence
of time effect (p = 0.041) with decreasing intrapersonal mean. There is no evidence
of interaction between time and profession (p = 0.71) (Figure
6).

Intrapersonal Variation Comparison Along Time

Mean (downwards) intrapersonal variation from
one measurement to the other was: SBP up to 6.7 mmHg (4.3 to 9 mmHg) with 95%
confidence interval, MBP up to 4.9 mmHg (3.1 to 6.7 mmHg) with 95% confidence
interval and MBP up to 3.3 mmHg (1.7 to 4.8 mmHg) with 95% confidence interval
being significant all mean decreases between 1st and 5th
measurements. These variations are device-independent (p > 0.25 for all pressures),
but may be profession-dependent. Mean decrease between 1st and 5th
SBP measurements was higher (17 mmHg) in instrumentation technician as compared
to other professions which were not different among themselves (p = 0.000);
MBP was lower in physicians (increase of 2.6 mmHg) as compared to other professions
which were not different among themselves (p = 0.009). Mean decrease from 1st
to 5th DBP measurements was not significantly influenced by profession
(p = 0.46).

Maximum difference in a same volunteer was 49
mmHg for SBP, 46 mmHg for MBP and 28 mmHg for DPB. All maximum variations were
observed from the 6th to the 8th minutes. No volunteer
presented the same value during the five measurements, showing an intrinsic
personal variation.

The 2-minute interval checking has shown wide
measurement variability and 25% of volunteers have complained about the tourniquet.

DISCUSSION

This study has shown that there are variations
in SBP, MBP and DBP measurements of healthy volunteers without history of arterial
hypertension. Maximum mean intrapersonal variation from one measurement to the
other has been up to 6.7 mmHg for SBP, up to 4.9 mmHg for MBP and up to 3.3
mmHg for DBP, with less measurement variation for SBP. Maximum intrapersonal
measurements difference has been 49 mmHg for SBP, 46 mmHg for MBP and 28 mmHg
for DBP, all between the 4th and the 5th measurement.
This way, SBP and MBP decreases are not the best parameter to evaluate blood
pressure in volunteers, as well as there are wide variations between 1st
and last measurements leading to intrapersonal errors.

Indirect blood pressure measurement technique
using arterial occlusion and stethoscope, according to Riva-Rocci 2,3
and Korotkoff 11 remains to date. Although this indirect blood pressure
reading is easy to teach and fast to perform, an automatic measurement is desirable,
especially in the operating room and intensive care unit, when repeated measurements
are needed. A major advantage of automatic devices is the possibility of ruling
out the influence of the observer. Blood pressure measurement is needed to handle
anesthetized patients. Noninvasive methods are routinely used due to their low
risk and simplicity. The interest in noninvasive mean blood pressure techniques
is increasing because this pressure is often evaluated by invasive techniques.
The oscillometric method started to be developed in 1973 and was placed in the
operating room in 1976 12. Since its introduction, several studies
have described the accuracy of devices evaluating mean blood pressure by the
indirect method in neonates, children and adults 13-17. Although
there are exceptions, several studies have shown that the accuracy of such devices
is below 5 mmHg of mean error with standard deviation of less than 8 mmHg as
compared to catheter insertion in the central artery. It has been observed an
intrapersonal variability from one measurement to the other of 4.3 to 9 mmHg
for SBP, 3.1 to 6.7 mmHg for MBP and 1.7 to 4.8 mmHg for DBP, regardless of
the device, showing that SBP and MBP are not a good parameter to evaluate minor
decreases in volunteers during a normal workday. In a same volunteer, SBP decrease
from one measurement to the other has reached 49 mmHg while MBP reached 46 mmHg
and DBP 28 mmHg, showing major intrapersonal variability during blood pressure
monitoring.

Several blood pressure treatment decisions during
anesthesia are made based on systolic and diastolic blood pressure. In severely
ill patients, mean blood pressure values are obtained through radial artery
puncture. It has been suggested that noninvasive monitors may perfectly replace
invasive blood pressure methods 18. A 14 mmHg MBP variation with
95% confidence interval has been shown in 19 patients 13. Our study
has found 4.9 mmHg variation for MBP with 95% confidence interval. Limit 80
mmHg value of the oscillometric monitor has shown a trend to overestimate blood
pressure above this limit and underestimate the evaluation below this limit
19. It has also been shown that the oscillometric monitor overestimates
blood pressure in the presence of deliberate arterial hypotension 19.
Correlation coefficients between invasive and noninvasive methods have shown
a good, however wide variability among individuals 20. Possible variations
among individuals may be explained by the width of the cuff used with the oscillometric
method because width equivalent to one third or half the arm's circumference
does not produce an error above ± 5 mmHg 21. This factor alone
does not explain variations among individuals. Cuffs used in our study were
within manufacturers' standards for adults, so have not contributed for the
wide measurement variations.

Arterial hypotension and bradycardia are the
most feared spinal anesthesia side-effects 22,23, especially in obstetrics,
because prompt and effective treatment is considered critical to prevent fetal
suffering. Arterial hypotension criteria used in most spinal anesthesia studies
published by the Brazilian Journal of Anesthesiology from 1999 to 2003 is 20%
8-10,24-26 to 30% 27-35 decrease of control SBP or SBP
below 90 mmHg 10. Traditionally, arterial hypotension in obstetrics
is defined as SBP decrease equal to or below 20% as compared to control, or
even, SBP below 100 mmHg, which should be monitored at 3-minute intervals 36.
Currently, blood pressure is monitored in more frequent intervals (every minute
till birth) and vasopressant therapy is early started to correct even 10% decrease
of control pressure 36. Noninvasive blood pressure devices take approximately
45 seconds for inflation, deflation and reading. The oscillometric method (as
compared to manual auscultation method) has a minor effect in measurement accuracy
when venous ingurgitation is not allowed to decrease. At least a little venous
flow should be allowed at the end of each measurement to avoid discomfort and
possible edema or petechia. In the normal automatic mode, when measurement time
is set for one minute, the shortest interval between measurements is approximately
25 seconds 25. In the immediate mode, time between measurements is
only a few seconds, but since these immediate measurements take approximately
15 seconds only, in general there is no discomfort or major venous ingurgitation
at the distal limb. However, to prevent any possibility of complications with
venous ingurgitation, the immediate mode should not be used more frequently
than necessary 37. In our study, 25% of volunteers have complained
of tourniquet during the 2-minute interval measurements.

The criteria of waiting for 20% decrease in systolic
blood pressure to administer therapeutic ephedrine doses in spinal anesthesia
for Cesarean sections, often recommended in the literature and adopted in the
clinic, is equally inadequate 8. So, some authors have proposed 38
and others agree 8 that the best way to treat arterial hypotension
is the therapeutic use of bolus vasopressants for any blood pressure decrease.
Bolus doses of 15 mg or higher than 2 mg.min-1 have determined high
incidence of arterial hypertension and prophylactic ephedrine has developed
arterial hypertension in 15% to 17.5% 9 or even unacceptable arterial
hypertension 8. Bolus doses equal to or higher than 15 mg, or prolonged
infusions equal to or higher than 4 mg.min-1 should not be used since
they may induce high incidence of maternal arterial hypertension or impair fetal
wellbeing 8. Blood pressure evaluation in volunteers at 2-minute
intervals has shown that we should be more careful in treating any systolic
blood pressure decrease because it may result in arterial hypertension, which
is as undesirable as arterial hypotension. No volunteer had the same value in
the five measurements, showing an intrinsic personal variation.

Mean blood pressure is less affected by vascular
tone changes than diastolic pressure, because it is obtained when oscillations
reach their highest amplitude during cuff deflation 25. This feature
allows mean blood pressure to be safely used, even in cases of arterial hypotension
with vasoconstriction and pulse pressure decrease 37. Conversely,
our study has shown that DBP presented fewer variations in volunteers not submitted
to surgical stress.

In evaluating three devices in volunteers aged
20 to 40 years, we have observed intrapersonal variations with significantly
different means of up to 6.7 mmHg for SBP, 4.9 mmHg for MBP and 3.3 mmHg for
DBP. So, treating any blood pressure decrease without taking into account mean
device error and intrapersonal variation may result in approximately 17% arterial
hypertension 9 or even unacceptable arterial hypertension 8
during spinal anesthesia for Cesarean section. The evaluation at 2-minute intervals
was uncomfortable for 25% of patients. This study has shown that DBP is the
best parameter to evaluate blood pressure changes in volunteers. Blood pressure
treatment during spinal anesthesia based on any SBP decrease should be revaluated.